1. Field of the Invention
The present invention relates generally to an anti-skid control system for an automotive hydraulic brake system. More specifically, the invention relates to a fail-safe system in an anti-skid brake control system.
2. Description of the Background Art
As is well known, an anti-skid brake control system adjusts braking pressure for maintaining wheel slippage close to an optimal level, i.e. 10 to 20%. For establishing this, it is usual anti-skid control operation to perform cycles of brake control operation to hold the braking pressure constant until the wheel slippage increases across a predetermined wheel slip criterion, reduce the braking pressure for resuming wheel speed and thereby recovering wheel slippage below the predetermined wheel slip criterion, holding the reduced braking pressure until wheel speed is increased to be coincident with a vehicular speed, and increasing braking pressure.
Because of an automotive brake system is essential element for assuring safe driving of the vehicle, various fail-safe systems have been developed and proposed for preventing the vehicle from falling into a dangerous state even when anti-skid brake control fails. For example, Japanese Patent First (unexamined) Publication (Tokkai) Showa 58-53551 discloses a fail-safe system for an anti-skid brake control system, which monitors a period of time in which the operational mode of the anti-skid brake control system is maintained in RELEASE mode to decrease the braking pressure, and detects the peroid longer than a predetermined possible maximum RELEASE period for detecting failure of the system. Though such fail-safe system generally operates well for preventing the vehicle from falling into danger, improvement is required for obtaining a further higher safety level.